feat: Replace python tests with native E2E test suite and fix capabilities schema

- Added native E2E test suite in stub to validate server<->stub<->nvim IPC
- Added win-nvim integration tests
- Fixed empty capabilities schema in initialize response in server and win-nvim
- Refactored server to accept dynamic MCP_PORT for test isolation
- Made server exit gracefully on port 3000 collision

gemini-ui.lua

@@ -60,6 +60,10 @@ local function execute_agent(prompt)
     
     M.show_progress("Starting Antigravity...")
     
+    -- termopen requires an empty and unmodified buffer
+    vim.api.nvim_buf_set_lines(progress_buf, 0, -1, false, {})
+    vim.bo[progress_buf].modified = false
+    
     -- termopen will attach to the current buffer (which show_progress just focused)
     job_id = vim.fn.termopen(cmd, {
         on_exit = function(_, code)
@@ -70,9 +74,16 @@ local function execute_agent(prompt)
 end
 
 function M.ask_agent()
+    local file = vim.api.nvim_buf_get_name(0)
     vim.ui.input({ prompt = 'Ask Agent: ' }, function(input)
         if not input or input == '' then return end
-        execute_agent(input)
+        
+        local final_input = input
+        if file and file ~= "" then
+            final_input = input .. string.format(" [Context file: %s]", file)
+        end
+        
+        execute_agent(final_input)
     end)
 end